1. Field of the Invention
The invention relates to a method for utilizing the thermal energy contained in subterranean geothermal formations to generate electricity, and particularly to an improved method for generating electricity using thermal energy derived from hydrothermal formations and dry hot rock formations.
2. Description of the Prior Art
Sources of geothermal energy are found in many parts of the world, including both dry hot rock formations and hydrothermal formations. While a few hydrothermal formations contain dry steam, which is particularly useful in the generation of electricity, and some contain wet steam, most of the known hydrothermal formations contain geothermal brine.
Because geothermal brines are usually corrosive and contain significant amounts of dissolved salts and suspended solids, past attempts to utilize these brines in the production of electricity have experienced only limited success due to the problems of scaling, plugging and corrosion of process equipment. Many of the more easily recoverable geothermal brines have relatively moderate temperatures, such as from 100.degree. F. to 350.degree. F. At present, electricity has not been commercially produced using such moderate temperature brines, and these brines have had little utility other than to supply simple process heating requirements.
Research is currently being conducted to develop methods for the production of useful energy by extracting thermal energy from dry hot rock formations. Since these formations are substantially free of naturally occurring fluids, a liquid working fluid is injected through a well or system of wells penetrating the formation wherein it is heated and/or vaporized to form a hot fluid, and the hot fluid is subsequently recovered for use in the generation of electricity. A major problem anticipated in the development of the dry hot rock formations as an energy source is that, due to the low permeability and low thermal conductivity of these formations, it is difficult to obtain a sufficiently large heat exchange surface in the formation to make the operation economically feasible. Explosive and/or hydraulic fracturing methods have been proposed to increase the surface area available for heat exchange, but the limited heat exchange surface obtainable is still believed to be a major impediment to the practical development of dry hot rock formations. Another problem is that the liquid working fluid often dissolves minerals from the dry hot rock formation and, if the working fluid is allowed to later flash to a vapor in the formation or in the production well, these dissolved minerals will be precipitated therein to reduce the formation permeability and/or the productivity of the well.
Thus, a need exists for a practical and economically viable method for extracting thermal energy from subterranean geothermal formations, especially from moderate temperature hydrothermal formations and from dry hot rock formations.
Accordingly, a principal object of this invention is to provide a method for economically generating electricity using thermal energy recovered from geothermal energy sources which have not as yet been successfully exploited.
Another object of this invention is to provide an improved method for generating electricity using thermal energy extracted from both a hydrothermal formation and a dry hot rock formation.
Yet another object of this invention is to provide a method for generating electricity which combines the extraction of energy from a moderate temperature hydrothermal formation and a dry hot rock formation, to thereby avoid the difficulties expected to be encountered in the separate extraction of energy from these geothermal formations.
Further objects and advantages of this invention will be apparent to those skilled in the art from the following description taken in conjunction with the accompanying drawing.